In a battery powered electronics device, such as a paging receiver, battery life, battery size and weight are among some of the most important considerations. Battery life, for a given size battery, is directly related to current drain and the minimum usable battery voltage from which the equipment will operate. The minimum usable battery voltage is referred to as end cell voltage.
The goal for a very small battery powered device has been to achieve single cell operation with long battery life. In keeping with this goal, it is desirable to design circuits that minimize current drain and that can operate to a very low voltage. Typically, an end cell operating voltage of 1.0 volts is specified.
In addition to the requirements mentioned above, the circuit must operate in a stable manner over the broad range of temperatures that the device will be exposed to when carried on a person or left in an automobile.
Analog integrated circuits are prime examples of circuits which benefit from the requirements discussed above. They require stable reference current sources and current mirrors for the biasing of various internal circuits and as references for analog to digital and digital to analog converters. Current sources and current mirrors designed using the present art have many characteristics that are inconsistent with the foregoing requirements. They have a high operating voltage that restricts the dynamic range of the signal that they can handle; they consume more current than is desirable; they use large geometry components; they have an undesirably low output impedance that affects the device accuracy; and they require complex ancillary circuits, such as startup circuits, to insure their proper operation.
Accordingly, what is needed is an improved current mirror and reference current source that are stable with temperature and supply voltage variations, consume little current, have a high output impedance and do not require additional supporting circuits for proper operation.